1. Field of the Invention
The present invention relates to a toner and a developer for use in electrophotography. In addition, the present invention also relates to a developer and an image forming method using the toner.
2. Discussion of the Background
In a typical electrophotographic apparatus or electrostatic recording apparatus, an electric or magnetic latent image is formed into a visible image by a toner. Specifically, in electrophotography, an electrostatic latent image is formed on a photoreceptor, and then developed with a toner to form a toner image. The toner image is transferred onto a recording medium such as paper, and fixed thereon by application of heat, etc. The toner for developing the electrostatic latent image typically includes colored particles in which a colorant, a charge controlling agent, and the like agents are dispersed in a binder resin.
As a method for fixing a toner image on a recording medium, a heat roller method is widely used due to its high energy efficiency. In recent attempts to reduce energy consumption in fixing, toners are required to be fixable at low temperatures. In other words, a smaller amount of energy is required when a toner image is fixed on a recording medium. The International Energy Agency (IEA) Demand-Side Management (DSM) program in 1999 involves a technology procurement project for next-generation copiers, and a requested specification is disclosed therein. Specifically, copiers with a printing speed of 30 cpm or more are required to have a warm-up time of 10 seconds or less and to consume energy in an amount of from 10 to 30 watts in the warm-up, which is a drastic energy-saving requirement compared to conventional copiers. To respond to the requirement, one possible approach involves reducing heat capacity of a fixing member such as a heat roller, so that temperature response of a toner is improved. However, this approach is insufficient to respond to the requirement.
To minimize the warm-up time, it is necessary that the melt-starting temperature of a toner is reduced so that the toner is fixable at low temperatures (this property is hereinafter referred to as “low-temperature fixability”). To respond to such a requirement, toners using polyester resins, which are fixable at lower temperatures and have better thermostable preservability than conventionally-used styrene-acrylic resins, have been proposed.
As described above, the heat roller method is widely used because of providing good heat efficiency and contributing to downsizing of an apparatus. In consideration of energy saving, the heat roller is required to consume less electric power.
To respond to such a requirement, fixing devices have been further improved recently. For example, the thickness of a fixing roller that contacts a surface on which a toner image is supported is reduced so that heat efficiency is increased. As a result, the warm-up time is drastically reduced. In this case, however, the specific heat capacity of the fixing roller is decreased, and therefore a difference in temperature between portions in which a recording medium passes or not may be large. Consequently, a hot offset problem occurs. The “hot offset” here refers to an undesirable phenomenon in that part of a fused toner image is adhered to the surface of a fixing roller, and re-transferred to an undesired portion of a recording medium. Accordingly, toners are required to have both low-temperature fixability and hot offset resistance.
To improve hot offset resistance of a toner, a release agent included therein is required to have a low melt viscosity and good separability from a binder resin. For example, Unexamined Japanese Patent Application Publication Nos. (hereinafter “JP-A”) 01-185660, 01-185661, 01-185662, and 01-185663 disclose toners including a carnauba wax and/or a montan wax, and Examined Japanese Patent Application Publication Nos. (hereinafter “JP-B”) 52-3304 and 52-3305 disclose toners including a hydrocarbon wax such as polyethylene, polypropylene, and paraffin.
However, hot offset resistance of these toners is yet insufficient. To make matters worse, in a case where the release agent is unevenly dispersed in the toner, chargeability and fluidity of the toner may be poor, thereby consistently degrading the resultant toner image. Therefore, the release agent is required to be finely dispersed in the toner, especially when the toner has a small size.